Effects of solder thickness on interface behavior and nanoindentation characteristics in Cu/Sn/Cu microbumps

The thickness of the solder, for Cu/Sn/Cu microbumps with dimensions of tens of microns or even a few microns (such as 40 µm, 15 µm, 10 µm, and 6 µm), can have a significant effect on the interfacial transfer and the performance. During the reflow stage, it was found that the grains of Cu6Sn5 showed “staggered growth” phenomenon in some areas of the microbumps with the solder thickness of 6 µm and the thickness of intermetallic compounds’ (IMCs) layer grew the fastest; that the second fastest growth of 40 µm; that the third fastest growth of 15 µm, and that the slowest growth of 10 µm. After thermal aging at 160 °C for 80 h, the thickness of the IMC layer in descending order was the microbumps with the solder thickness of 15 µm, 10 µm, 40 µm, and 6 µm. These results were caused by the difference in the element concentration. In addition, there were four main components in the microbumps: Cu, Sn, Cu6Sn5, and Cu3Sn. The hardness of Cu and Sn was about 1.37 ± 0.3 Gpa and 0.13 ± 0.03 Gpa, respectively. And the hardness of Cu6Sn5 and Cu3Sn was about 6.7 ± 0.3 Gpa and 6.2 ± 0.4 Gpa, respectively. The nanoindentation hardness analysis showed a general increase in the hardness of the microbumps after thermal aging for 80 h, compared with that after reflow. And the thinner the solder thickness was, the more obvious the hardness improvement was.